Spin labels specifically attached to sites on the myosin regulatory light chain have helped in elucidating the conformational changes involved in the myosin force generation cycle. However, probes attached to the native cysteines in wild-type light chains have so far been less than ideal, due to mobility or disorder with resultant loss in resolution and sensitivity. the facilities of the Computer Graphics Laboratory and MidasPlus, we have designed and subsequently expressed several mutant light chains, placing novel cysteines at nw locations. Also, monofunctional probes have been placed where modeling indicates the spin labels may be well oriented and immobilized. We have also used MidasPlus to design and model bifunctional spin probes, determining the likely orientation of the probes graphically.